1. Field of the invention
The present invention relates to rotary furnace for indirectly heating materials by utilizing combustion gas of fuel.
2. Description of the related art
One of the most efficient and economical methods for heating powder or granular materials is burning fuel generate high temperature gas and the subjecting the materials to heat exchange with this gas. The combustion gas may include gaseous components which are capable of reacting with the materials at high temperature. In this case, the above method cannot be utilized for heating, not withstanding the efficiency. In order for heating the materials which are capable of reacting with the combustion gas, electricity must be used as a heat source instead of fuel, or in another case, inert gas must be introduced in a furnace. As a result, economy of heating is disadvantageously impaired.
The oxidizing gaseous components, such as oxygen, carbon dioxide, hydrogen oxide and sulfur trioxide, are contained in the combustion gas of fuel.
When ore is heated under such as oxidizing atmosphere to reduce the same, ore is liable to be exposed to such oxidizing atmosphere. This is the very reverse of what is intended by heating. The reducing method of ore by heating it in a rotary kiln by means of combustion gas of the fuels, such as coal, heavy oil and LPG, is broadly used for smelting, since inexpensive energy can be use, and further, continuous treatment by mass production is possible. However, the combustion gas includes, as described hereinabove, oxidizing gaseous components, such as exessive oxygen, carbon dioxide, hydrogen oxide and sulfur trioxide, with the result that the atomosphere of combustion gas is not the objective reducing one but is the oxidizing one, which is unsatisfactory in the light of the increasing reduction degree.
To isolate the materials to be reduced from the oxidizing atmosphere combustion gas, it is known to enclose the combustion flame in a ceramic tube in order to heat the materials indirectly through the ceramic tubes by utilizing radiation and conduction of the heat. For example, U.S. Pat. No. 1,871,848 discloses an isolation method (CF. FIG. 3) mentioned above.
Another method for isolating the materials to be reduced from the oxidizing atmosphere of combustion gas, is to apply a coating layer on the surface of the materials to be reduced. In this case, the materials are substantially heated in unoxidizing atmosphere. Such a method is disclosed in the U.S. Pat. No. 3,153,586.
The method disclosed in the U.S. Pat. No. 1,871,848 mentioned above involves a problem that mechanical strength of the ceramic tube is decreased at high temperature. It is difficult to manufacture pipes having a large diameter and length. The highest temperature at which the furnace disclosed in above-mentioned U.S. Pat. No. 1,871,848 operates is 1,000.degree. C. at the highest. Iron ore is only one ore that can be reduced at this temperature. The greatest length of pipe that can be manufactured is 2.about.3 m at the most. It is impossible to entirely surround the combustion flame by such as pipe, and hence to effectively isolate by such a pipe the materials to be reduced from the combustion gas of fuel. Such method as disclosed in the above-mentioned patent is therefore not appropriate for reducing ore which contains, such metal as chromium, having high affinity with oxygen, and which is liable to be influenced by the atmosphere of combustion gas.
It is an object of the present invention to improve a rotary furnace constitution having high treating capacity, and having an ability that materials to be treated are effectively isolated from the combustion gas of fuel.